In high speed position, you should have the following measurements at the high speed blower relay, with the relay plugged in:
Primary side of the relay:
12 volts coming in on the orange wire from the switch
good chassis ground at the black wire
Secondary side of the relay:
12 volts coming into the relay on the red wire from the power distribution junction block
12 volts out on the purple wire to the blower motor.
The 5th wire, dark blue in color, leads back to the blower resistor block and is not used in high speed operation, only the other speeds. When the relay is denergized, the default contact position on the secondary side connects the purple wire to the dark blue wire; this connects the resistor block into the power circuit to the motor, alowing lower speeds to be obtained by switching various resistors into the circuit.
When the switch is placed in HIGH speed position, the switch supplies power to the relay primary side via the orange wire. This energizes the relay, which then switches the purple wire connection from the dark blue one to the red one instead (direct battery power, bypassing all the resistors allowing for maximum speed).